By Eli Sagor, UMN Extension
Few issues have dominated forestry in Minnesota more than the future of ash trees threatened by emerald ash borer. With about a billion trees on almost a million acres in Minnesota alone, ash is important culturally, economically, and ecologically here. The EAB threat has led to two very different areas of research and Extension education: with communities preparing for changes to their urban forests and with foresters concerned about future health and productivity of their lands.
In August I spent a day in the woods north of Deer River for an update on this second line of research. I joined a group of foresters learning about silvicultural options to maintain resilient and productive black ash stands under threat from EAB. As the third in a series on ecological silviculture in pine stands, mixed stands, and now ash, this post reviews the research presented there as well as the opportunities and issues discussed by the land managers present. The workshop was led by Tony D’Amato of the UMN Department of Forest Resources and John Almendinger and Dan Hanson of the Minnesota DNR. It was offered by the University of Minnesota’s Sustainable Forests Education Cooperative.
Emerald ash borer is an introduced beetle that causes nearly 100% mortality on North American native ash trees. EAB is now established in several counties in Minnesota, as well as a few miles from Duluth in Superior, Wisconsin. Because our native ash have no resistance to EAB and at least some may survive Minnesota’s cold winters, there is concern that we will see ash mortality across the north woods. This causes obvious concerns not only about economic, but also wildlife habitat, human cultural, and other impacts.
Ash is widely distributed across northern Minnesota forests and woodlands. While it is found on uplands as well, most ash exists on wet forest sites that are inhospitable to most other native tree species. Long-term survival of other species on ash sites tends to be limited primarily by moisture. Adding to the concern about loss of ash is the potential that these sites will become even wetter if the existing ash canopy is lost, which would reduce evapotranspiration (removal of moisture from soil through the roots). This could make these sites even less hospitable to other species, prompting concern about long-term loss of any forest cover on these sites. This potential is discussed in Extension’s 2011 Ash Management Guide‘s section on wet forests.
The focus of the day was less on EAB and more on strategies to maintain healthy, resilient, and productive forests across northern Minnesota before, during, and after any impacts associated with EAB.
Ash stands visited: Wet and very wet
We visited two different types of ash stands: Northern wet ash swamp and northern very wet ash swamps. (For technical details, see MN DNR ecological classification native plant community fact sheets WFn55 and WFn64, respectively.) Wet ash swamp sites typically have standing water only into late spring, while very wet ash swamp sites may have standing water through spring and summer.
While difficult to depict in the photos, the very wet ash swamp that we visited first had the hummocky topography typical of these stands. Trees were virtually 100% black ash and while there was little standing water due to a dry late summer, our feet were certainly wet as we left the site. The virtual exclusivity of black as on this site was attributed to that species’ ability to withstand year over year of repeated inundation to a degree that would do in most other species that commonly occur on wet sites.
Next, we crossed the road to visit a slightly less wet as swamp. Conditions were quite different here, with more species diversity, less hummocky topography, and drier soils. We discussed silvicultural options at some length at this site. The increased species diversity of course created more opportunities for a gradual replacement of ash prior to arrival of EAB, but the concern was that a sudden loss of ash could lead to ponding, causing this stand to behave more like the very wet swamp we visited first.
Silviculture: Black ash research plots
The day’s focus was on maintaining healthy, resilient, and productive forests and woodlands in northern Minnesota before, during, and after EAB arrives. Resilience refers to the system’s capacity to return to a functioning order after disturbance. The focus was on strategies to reduce the likelihood of ponding to a degree that would lead to long-term loss of forest on the site. Working with staff at the Chippewa National Forest, UMN silviculturist Tony D’Amato has installed research plots in ash stands to find out how different kinds of treatments affect future stand development.
Plots were installed in the dryer of the two plant communities discussed above. In other words, prior to the treatments standing water would be present into spring, but generally not beyond.
First we visited a four-acre clearcut, harvested about 18 months before our visit. Conditions varied somewhat across the stand, with some areas regenerating nicely and others inundated enough that they became dominated by lake sedge and unlikely to return to forest cover soon. In general, the view was that clearcutting carried a substantial risk of excessive ponding and long-term loss of forest cover.
Girdling ash to simulate EAB infestation
Next we entered research plots in which all of the standing ash had been girdled to simulate mortality from EAB invasion. Girdled trees were gradually dying, likely over a period of 2-3 years. This gradual pattern led to less of a hydrologic shock to the system, and less ponding. There was good regeneration from a variety of species under the partial shade caused by the thin ash crowns.
Group selection refers to the removal of standing trees in groups, leaving relatively small gaps. Group (and gap) size can vary based on owner objectives, site conditions, and species targeted for regeneration. In these plots, gaps were relatively small, at only 1/10 acre (80 feet) across. While it is still early, the group selection sites appear to be regenerating well also.
Trees grow slowly and forests will develop and change over decades. With these treatments still only 18 months or so old, it is a bit early to draw firm conclusions. However, early results, as well as observation of other sites, lead to a few general conclusions:
- The risk of ponding and long-term loss of forest cover is real, and can be a consequence of rapid mortality on wet sites. This should be avoided if possible. This reinforces a common recommendation that landowners should not overreact and clearcut all ash out of concern over EAB. A more patient, stepwise approach may lead to better outcomes.
- Maintaining some canopy cover is strongly recommended in order to avoid excessive ponding.
- Harvesting up to half of the canopy, either uniformly or in small gaps, can stimulate regeneration while reducing the risk of ponding and loss of tree cover.
- Whatever the harvesting pattern, maintaining continuous canopy cover in wet ash stands is essential.
The bottom line
While EAB does threaten native ash, landowners and managers should focus on maintaining healthy, resilient, and productive stands and not focus solely on one species. Maintaining a variety of species and age classes is an important strategy to foster resilient stands. Landowners should proactively begin to prepare ash stands for the arrival of EAB, but should avoid overreacting by removing all ash in a single treatment. A better approach would be to begin a series of entries designed to remove less than half of the canopy at a time, thereby promoting new regeneration while avoiding major hydrologic changes.
This August 2013 workshop was organized by Mike Kroenke of the University of Minnesota’s Sustainable Forests Education Cooperative. All of the tour stops were on Chippewa National Forest lands. It was one of a series of field days focused on silvicultural applications grounded in Minnesota’s Ecological Classification System. More about the University of Minnesota’s silviculture research lab.